In the last decade or so, considerable effort has been expended to improve the fuel efficiency of vehicles. This effort has resulted in virtually every component of a vehicle being reviewed for its effect on fuel economy. In general terms, the main focus has been on the weight of the component and on the effect of a component on vehicle aerodynamics, that is, on the drag coefficient of the vehicle.
In many instances, a given component will affect one or the other, but not both, of the above considerations. In other cases, a given component can have an effect on both. Examples in the latter category include many of the heat exchangers employed in a vehicle as, for example, radiator and air-conditioning system components.
Such components add weight to the vehicle and to the extent that their volume may affect the frontal area or some other part of the vehicular envelope, the drag coefficient as well. Consequently, there is an ongoing effort to not only reduce the weight of such heat exchangers as by switching to light weight components, but to reduce the volume as well since such will not only be accompanied by a reduction in weight, but may permit a reduction in some part of the envelope of the vehicle which allows a reduction in the drag coefficient.
At the same time, the heat exchange capabilities of a system including such components does not diminish and may even increase. Consequently, a reduction in the size of the heat exchanger which would, all other things being equal, reduce the heat exchange capability of the system in which it is used, must be offset or compensated for by increased efficiency. For if efficiency cannot be improved, in order to meet the heat exchange requirements of a given system, desired size reductions cannot be accomplished and weight reduction efforts will be limited to the availability of lighter weight components that may be substituted for those currently in use.
While over the years there have evolved a number of set techniques that may tend to increase heat exchanger efficiency, in many respects, attempts to use these techniques to obtain greater efficiency in refrigeration or air-conditioning systems simply are not successful because of the fact of two-phase flow in a system heat exchanger such as the condenser or evaporator. That is to say, because heat exchangers of these types are dealing with a refrigerant that will be partly in the liquid phase and in part in the vapor phase, techniques that may produce an increase in efficiency in single phase units such as vehicular radiators are inapplicable. And even condensers and evaporators behave quite differently from one another and require assessment of different situations. For example, in an evaporator, condensate from humid air passing through the evaporator core will form on the core itself. The condensate is a far poorer thermal conductor than the aluminum components of which evaporators for vehicles are typically formed. As a consequence, if condensate is permitted to remain on fins and tubes of an evaporator, since heat must pass from the ambient air to the fins and tubes through the condensate before evaporation of the refrigerant can take place, the condensate impedes heat transfer and thus reduces efficiency. Furthermore, particularly with relatively high fin densities, the condensate may block air flow through the core. With reduced air flow, efficiency is further reduced.
And, in worst case situations, the condensate may actually freeze to solidify the poor thermally conducting layer surrounding the fins and provide an immovable obstacle to the passage of air through the core.
Refrigerant flow through an evaporator is also of concern. If a greater mass of refrigerant is flowing through a specific part of an evaporator, a cold spot will result. Such a cold spot can result in condensate freezing at that location. In the alternative, the existence of a cold spot is indicative of the remainder of the evaporator not performing at optimum efficiency.
The present invention is directed to solving one or more of the above problems.